This invention relates to radar targets and more particularly to a novel method and means for controlling the amount of reflected electromagnetic energy that such a target will return.
Radar target cross section reduction has long been used as a tactical device for making the detection of aircraft and missiles difficult for enemy search radar. Such a technique is also useful in discriminating against the non-preferred of two or more closely situated antennas and in diminishing the scattering effect of antenna support structure. Radar target cross section enhancement also has several practical applications. Radar beacons, IFF systems and air traffic control systems are examples.
The electronics industry has consequently been active in developing techniques for accomplishing control of radar target cross sections. The bibliography entitled, Articles on Radar Reflectivity, by Corriher and Pyron, published in the IEEE Proceedings, Vol. 53N8, August 1965, pages 1025-1065, is an exhaustive review of the state of the art through 1965.
Initial attempts to achieve radar target cross section reduction were directed toward the use of absorbing materials. Such absorbing materials, however, are not very satisfactory since they have little effect when the radar wavelength is larger compared to the target dimensions or the target is observed by a forward scattering radar whose wavelength is small compared with the target dimension. Further, absorbing materials add unwanted weight to missiles and in general cannot withstand reentry temperatures.
The possibilities of controlling radar cross sections of objects by placing the equivalent of properly adjusted small lumped impedances at strategic points in the surface of the object have been investigated and the feasibility of such an approach has been clearly demonstrated. However, with this technique, which is called impedance loading, resulting reduction or enhancement of the scattered energy has been quite narrow-band because the required loading reactances must either be nearly constant or have a negative slope as the frequency is varied, and this type of behavior cannot be obtained with the usual linear, passive, bilateral, lossless elements.
Two basic methods have been used for increasing the bandwidths. The first is a direct synthesis of the required loads using active or nonlinear elements. This approach is difficult because synthesis procedures for these elements are not well established. The alternative method uses a device that senses the incoming frequency and tunes the impedance to a correct value to minimize the scattered energy for that frequency. In effect, this method moves a fairly narrow-band radar cross section control window over a wide range of frequencies. Previous forms of this method have succeeded in obtaining reductions of only two or three db in the scattered energy because of excessive losses in the tuning elements.
The present invention is directed toward overcoming these and other limitations that are prevalent in the state of the art radar target cross section control devices.